Vehicle engines may be configured to operate using diesel fuels. However, at low ambient temperatures, such as during an engine cold-start, wax may precipitate out of the diesel fuel. The precipitated wax may clog a fuel filter while also reducing the fluidity of the fuel. The amount of wax that precipitates from the fuel may depend upon the fuel properties and ambient temperature the vehicle is started in. As such, the precipitated wax in the fuel reduces the pressure of the low pressure fuel system and performance of the high pressure fuel system and, if severe enough, can cause damage to the fuel system.
Various strategies may be used to reduce and remove the precipitation of wax from diesel fuels. In one example approach, the mode of operation of a supply pump that pressurizes fuel and supplies pressurized fuel to the fuel rail and injectors may be adjusted responsive to the fuel temperature. As such, the supply pump may be operated in a more power-intensive pressure-controlled mode or a more energy-efficient volume-controlled mode. In the aforementioned example approach, the pump may be operated in the pressure-controlled mode until the fuel temperature reaches a threshold temperature to address the wax in the fuel. In another example approach, fuel heaters may be used to heat the fuel and address the issue of wax build-up.
Yet another example approach is illustrated by Osaki in EP 1,319,821 A2. In EP '821, an engine fuel system is configured with a fuel recirculation passage for recirculating heated fuel, discharged by the supply pump, through a fuel filter. Specifically, during engine cold-starts, when the temperature of the fuel drawn into the supply pump is lower than a threshold temperature, the quantity of fuel flowing through the recirculating passage is increased while the quantity of fuel returned to the fuel tank may be reduced.
However, the inventors have recognized several potential issues with such approaches. As one example, there may be a trade-off between increased fuel economy and ensuring sufficient wax removal. Specifically, when adjusting the supply pump operation mode responsive to fuel temperature, if the temperature is set low (for example, to improve fuel economy), there may be insufficient wax removal during some conditions. Alternatively, if the temperature is set higher (to ensure sufficient wax removal), under some conditions this may prolong the high work load of the pump more than necessary, adversely impacting fuel economy. For example, a fuel system can be operating in pressure control mode even though wax has been removed, even during low temperatures. Thus, continuing high work loading of the pump may be unnecessary. Further still, variations in the fuel quality and properties may significantly affect how much, and at what temperatures, wax is formed or removed.
Thus in one example, some of the above issues may be addressed by a method of operating a fuel system in an engine, the fuel system including a supply pump for pressurizing fuel received from a low pressure feed pump, a fuel tank, a fuel filter for filtering fuel, a fuel rail, and a fuel injector. One example method comprises, during an engine cold-start, operating the supply pump, and adjusting the operation mode between at least a pressure-controlled mode and a volume-controlled mode based on a fuel temperature and pressure.
In one example, the operation of the supply pump in a vehicle fuel system may be adjusted responsive to both a temperature and pressure of the fuel. Herein, at the onset of an engine cold-start and during engine operation in cold ambient conditions, the supply pump may be operated in the more power-intensive pressure-controlled mode for an interval to rapidly raise the fuel temperature to a first threshold temperature and pressure. Once the threshold temperature and pressure have been attained, the supply pump operation may be switched to the more energy-efficient volume-controlled mode. By operating the supply pump responsive to both a temperature and pressure of the fuel, substantial fuel economy benefits may be achieved. For example, the combination of a higher pressure threshold and a lower temperature threshold may suffice to enable improved wax removal and increased fuel fluidity. In some examples, the higher pressure and lower temperature combination may entail a shorter pump operation, thereby reducing the fuel consumption of the vehicle.
Further improvements in fuel fluidity and wax removal may be achieved by recirculating at least some of the fuel pressurized and heated through the supply pump into the inlet of one or more system fuel filters along respective recirculation passages. Flow through the recirculation passages may be regulated by respective one or more thermal recirculation valves. As such, the heated return fuel may be re-circulated irrespective of the operating mode of the supply pump. The amount of return fuel re-circulated through the fuel filters may also be adjusted responsive to the fuel temperature and/or pressure by regulating the flow of return fuel through the thermal recirculation valves. During operation of the pump in the volume-controlled mode, the build-up and removal of wax at the fuel filter may be diagnosed by analyzing the pressure at the filter outlet. By adjusting the recirculation flow based on a fuel temperature and/or pressure, wax removal may be expedited and the quality of fuel injection may be improved. Furthermore, by diagnosing and distinguishing filter clogging due to wax build-up from filter clogging due to extraneous matter (i.e. contaminants such as dust), the quality of engine and pump operation may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.